1. Field of the Invention
The present invention relates to the field of semiconductor processing and more specifically to a method and apparatus for dissolving a gas into a liquid used for cleaning, etching, or rinsing a wafer.
2. Discussion of Related Art
Wet etching and wet cleaning of silicon wafers is usually done by immersing the wafers into a liquid. This can also be done by spraying a liquid onto a wafer or batch of wafers. Wet wafer cleaning and etching is traditionally done in a batch mode where several (e.g., between 50–100) wafers are processed simultaneously. Because of the need a short cycle time in chip manufacturing, there is a need for fast single wafer processing. In wet processing the amount of dissolved gas in the liquid used for processing is very important for process results.
When performing wet etching operations, the amount of gas is very important for process results. At first there are the common reactive gases such as O2, O3, and H2. On the other side there are the inert gases such as N2, Ar, and He. Both the reactive gas content and inert gas content are important in wet processing. The reactive gas can react with the semiconductor wafer surface or with contaminants on the surface. The inert gases determine the amount of cavitation during a megasonic cleaning step and therefore influence the particle removal. It is very important to control the amount of both reactive gases and inert gases dissolved into liquids when using wet processing to treat semiconductor wafers. Sometimes it is assumed that all dissolved reactive gases are “bad” and therefore it is assumed that the concentration of O2, O3, and H2 has to be reduced to the minimum level possible. However, the right amount or the absence of reactive gases is dependent on a particular process and there is not a general level of dissolved reactive gases that is adequate for every process step. Present methods for dissolving gases into liquids include membrane contactors, bubbling chambers, spraying methods and shower type methods. A problem with these methods and apparatuses, however, is that they are bulky and expensive. They are ideal for batch processing tools but are to large and to expensive for very small single wafer tools.
Thus, there is a need for a simple and inexpensive way for dissolving the correct amount of reactive and inert gases into etching, cleaning, and rinsing liquids.